Browsing by Author "Lian Xie, Committee Chair"

Now showing 1 - 9 of 9

Adapting the Weather Research and Forecasting Model for the Simulation of Regional Climate in East Africa
(2007-04-22) Zhang, Xuejin; Lian Xie, Committee Chair
The regional climate model (RCM) is a tool to fill the gap between the outcome of global climate model and the demand of regional government and society for regional climate information. The current Weather Research and Forecasting (WRF) RCM inherits several advantages of the original WRF model. For example, (1) it can be used for multiple scale applications from large eddy to planetary scale; (2) it can be coupled to other climate system component models; (3) it has multiple physical options for different purposes of research; and (4) it has parallel infrastructure to distinguish the scientific problems from engineering problems. In order to adapt WRF for long term integration for climate application, the model and its pre- and post-processors are modified in several aspects, including SST and other surface parameters treatment, and exponential relaxation lateral boundary. In this study, we also transferred a pre-released version of CAM3 radiation scheme that was first implemented into WRF by NCAR Regional Climate Modeling team to the WRF standard release versions (2.1.1 released as of November 8, 2005 and 2.1.2 released as of January 30, 2006). The preliminary investigation demonstrates that the model has shown the encouraging and promising results on simulating regional climate. The objective of this study is to investigate the hydrological cycle in the East Africa region. Through data analysis, the water channel in the East Africa region is determined. The moisture transport from the Indian Ocean is the main source of the water vapor for precipitation in "short rain" season from October to December (OND). The variability of the moisture transport directly results in the variability of precipitation in OND. The local soil moisture, which directly connects to evaporation, does not substantially contribute to precipitation in the region. To investigate the physical processes of the variability of the moisture transport, five seasonal simulations from September to December for 1994 to 1998 are conducted by WRF RCM. The model successfully simulates the interannual variability of two positive Indian Ocean Dipole Mode (IODM) episodes (1994 and 1997), two negative IODM episodes (1996 and 1998), and the meridional mode in 1995. The analysis further demonstrates that the model can reproduce many surface features of the interannual variability related to IODM changes, such as the mean sea level pressure, wind regime, and surface air temperature. The seasonal change of the monsoon system is also well represented. The results show the interannual variability in East Africa and the adjacent region is mainly related to the variability of the Indian Ocean SST. Through sensitive experiments of different resolution SST forcing, we suggest that the SST forcing itself and SST gradient plays a role on exerting effect on atmosphere. The current conclusions are very preliminary and the results should be further carefully examined in future publications.
Analysis and Modeling of Wave-current Interaction
(2006-11-03) Liu, Huiqing; Lian Xie, Committee Chair; Gerald S. Janowitz, Committee Member; Leonard J. Pietrafesa, Committee Member; John M. Morrison, Committee Member
The main task of this study focuses on studying the effect of wave-current interaction on currents, storm surge and inundation as well as effects of depth-induced wave breaking, wind field and current on waves by using numerical models. The results show that it is important to incorporate the wave-current interaction effect into coastal circulation, storm surge and inundation models. At the same time, it should consider effects of depth-induced wave breaking, wind field, currents and sea surface elevation in prediction of waves. Specially, we found that: (1) Depth-induced wave breaking plays an important role in wave field in shallow water areas; (2) To more properly model the hurricane induced wave field, it is important to consider the asymmetric structure of the hurricane wind field, the changes in the hurricane translation time history, and the incorporation background wind field into hurricane wind field; (3) For SWH, it will be decreased when waves propagate in the following current direction. On the other hand, current will increase the SWH when waves propagate countercurrent direction. The change of wave propagation direction after crossing Gulf Stream depends not only on refraction, but also on others (e.g. trap waves effect). (4) It is important to introduce wave-current effects into any storm surge and inundation prediction modeling system. Specially, the consideration of wave-induced wind stress, bottom shear stress, and 3-D radiation stress in storm surge and inundation modeling can significantly improve the correctness of the prediction.
Analysis and prediction of Atlantic tropical cyclone activity
(2008-11-09) Keith, Elinor Whitney; Lian Xie, Committee Chair; Anantha Aiyyer, Committee Member; Montserrat Fuentes, Committee Member
This work begins with the development of a statistical prediction model of numbers of tropical storms, hurricanes and major hurricanes per year in several regions of the Atlantic: the entire Atlantic, the Caribbean Sea and the Gulf of Mexico, as well as landfalling storms along the US Gulf of Mexico, Southeast and Northeast coasts. The methodology involves of cross-correlating variables against Empirical Orthogonal Functions (EOFs) of the Hurricane Track Density Function (HTDF) to select predictors. The model performs well in the basin-wide predictions over the entire Atlantic and Caribbean, with the predictions showing an improvement over climatology and random chance at a 95% confidence level. Over the Gulf of Mexico, only named storms showed that level of predictability. Predicting landfalls proves more difficult, and only the prediction of named storms along the US Southeastern and Gulf Coasts shows an improvement over random chance at the 95% confidence level. Tropical cyclone activity along the U.S. Northeastern Coast is found to be unpredictable in this model; with the rarity of events, the model is unstable. In order to provide some physical basis for many of the connections found statistically, the second section is a case study of the 2004-07 Atlantic hurricane seasons. It is found that 2005 had the most favorable SST and vertical wind shear conditions over the main development region. 2004 and 2006 had intermediate levels of SST and wind shear and, outside of the month of August, similar levels of activity. Activity in 2007 was generally suppressed: although more tropical storms formed than in 2006, they were very short-lived. On average, tropical storms in 2007 survived less than 2.5 days. The strength of the subtropical anticyclone is a very important factor: in 2005, a weak subtropical high allowed for unusually high SST in the main development region, while in 2007 a strong subtropical high over the east Atlantic cooled SST and increased vertical wind shear. The strength of the subtropical cyclone may be related to the heat release of the African monsoon. This finding also emphasizes the importance of factors relating to the strength of the subtropical high pressure in hurricane prediction.
Cape Fear River Estuary Modeling System
(2008-02-28) Xia, Meng; Leonard J. Pietrafesa, Committee Member; Lian Xie, Committee Chair; Daniel L. Kamykowski, Committee Member; John M. Morrison, Committee Member
The Cape Fear River Estuary (CFRE) region is one of the coastal regions facing frequent threats from tropical cyclones. It is also an important nursery for juvenile fish, crabs, shrimp, and other biological species. Thus, predicting the physical responses of the CFRE system to extreme weather events is important to the protection of life and property and ensuring the economical well beings of local residents. In this study, the Princeton Ocean Model (POM) is used to simulate the storm surge circulation in the CFRE and adjacent Long Bay and Onslow Bay. The Environmental Fluid Dynamic Code (EFDC), an estuarine and coastal ocean circulation model, is also used to simulate the salinity plume and tracer plume distribution and particle trajectory in the vicinity of the mouth of the Cape Fear River Estuary (CFRE). The effects of astronomical tide, river discharge and wind on the CFRE salinity plume and tracer plume, particle trajectory were investigated. The comparison among the plume structure, particle trajectory, and the passive tracer structure is discussed. To better simulate the plume structure in Cape Fear River Estuary (CFRE), we also test the sensitivity of the EFDC model to the choice of grid resolution, advection scheme, and external forcing.
Interannual Variability of Climatology and Tropical Cyclone Tracks in North Atlantic and Western North Pacific
(2006-05-01) Yan, Tingzhuang; Lian Xie, Committee Chair; John M. Morrison, Committee Member; Fred H. M. Semazzi, Committee Member; Leonard J. Pietrafesa, Committee Member
The spatial-temporal variability of tropical cyclone tracks and their possible association with tropical cyclone landfall frequency along the United States East Coast and China East Coast are studied using Principle Component Analysis of tropical cyclone Track Density Function (TDF). Results show that North Atlantic (NA) hurricane TDF is strongly modulated by El Niño-South Oscillation, the tropical Atlantic SST dipole Mode (DM), North Atlantic Oscillation and Arctic Oscillation. Dominant Modes of Western North Pacific (WNP) typhoon TDF demonstrate strong correlation with spring and winter snow cover (SC) over the Qinghai and Tibetan Plateau (QTP). Results provide a foundation for the construction of statistical models, which project the annual number of tropical cyclone landfall along the East Coast of the United States and the coast of China. Analysis for 1990 and 2004 NA hurricane seasons revealed that the substantial variability of tropical Atlantic SST DM is a dominate factor affecting the hurricane track patterns. Study for 1978 and 2001 typhoon cases in the WNP demonstrated that the QTP SC was responsible for the differentiation in the number of landfall typhoon events in the WNP. A schematic diagram was proposed to illustrate the linkage between the DM and the NA hurricane track patterns. Accumulated gain or deficit in the surface radiation associated with the QTP SC imposes a long memory in the East Asian climate system. Variations in heat budget change the large-scale zonal circulation and further modulate the seasonal position and strength of East Asian subtropical high. A possible physical link to connect the QTP snow cover and the WNP typhoon track patterns was therefore proposed.
Intercontinental and Regional Modeling of Air Pollutants Transport over the Pacific Regions
(2006-08-10) Wang, Binyu; Yang Zhang, Committee Member; Carey Jang, Committee Co-Chair; Lian Xie, Committee Chair; Viney Aneja, Committee Member
Fine particulate (PM2.5) continues to be a major air pollution problem due to its adverse effects on human health and the environment. Recently, the rapid industrialization now taking place in Asia is producing a large and growing quantity of NOX, SO2, CO and other atmospheric emissions. This research is part of the work of project "Intercontinental Transport and Climatic Effects of Air Pollutants" (ICAP) sponsored by U.S. Environment Protection Agency (EPA). It presents modeling and interpretation of the impact of Asian anthropogenic emissions on air quality in the U.S. through long distance transport of Asian air pollutants. In order to better quantify the magnitude and impacts of Asia anthropogenic emission on U.S. air quality caused by intercontinental transport, a 3-D regional air quality model, the EPA Models-3 Community Multiscale Air Quality (CMAQ) system has been employed in this study. The model is applied to both baseline simulation of 2001 and the sensitivity simulation in which anthropogenic Asian emissions were turned off. The baseline simulation was evaluated with both surface data over U.S. and aircraft observation over Western Pacific regions. Compared to surface measurement over U.S., over 50% of the simulated PM2.5 concentrations fall within a factor of two of the observations in all seasons except winter. Model evaluation using vertical aircraft data indicates that the model is able to well reproduce the vertical distributions of SO42-, NO3-, NH4+ and CO. Concentrations of all species decrease rapidly with increasing altitude below 6 km, followed by a much slower decrease between 6 and 12 km. Time series analysis indicates that model can place the plume in the right location, but tends to under-predict the peak values of the plume intensity. This study shows that the impact of Asian man-made emissions is persistent throughout the entire year rather than episodic. There also exists a significant seasonal cycle with an April/May maximum (1~1.2 μg/m3 by average) and a July/August minimum (0.2 ~ 0.3 μg/m3 by average) for both eastern and western U.S. despite of the fact that Asia has the monthly average maximum for PM2.5 in January. During the simulated year, SO42- is the major component of Asian pollutants in each month for both eastern and western U.S. Cross section study along 140oE indicates although the concentration of PM2.5 species has a maximum value below 1 km, while distribution of eastward fluxes of Asian aerosol species shows the strongest flux at 2-4 km due to strong westward wind flow. By integrating the flux at different layers along ~140oE, it turns out that maximum flux of PM2.5 is at 35-45oN in the boundary (< 2,500m) and at 20?35oN in the free troposphere (2,500 -16,000 m). Further study on the transport pattern of SO42- and NO3- indicates that SO42- is transported mainly from Asia directly to the U.S., while NO3- may be transported through transport of its precursors (mainly PAN). We also compare the model results over the continental U.S. domain using three different continental influxes; one from a hemispheric-scale CMAQ simulation, and the other two from a global model GEOS-CHEM simulation and from static boundary conditions. The simulation with boundary conditions (BCs) provided by the CMAQ modeling has a better model performance in the western U.S. than the GEOS-CHEM and static BCs when compared against CASTNet observation. While further studies on other species are needed, our results suggest that CMAQ modeling can be a suitable tool to address intercontinental transport of air pollution, an issue of hemispheric scale in nature that has been previously addressed by field campaigns or global models.
Modeling Tropical Cyclone Induced Inland Flooding at Tar Pamlico River Basin of North Carolina.
(2010-07-27) Tang, Qianhong; Lian Xie, Committee Chair; Fredrick Semazzi, Committee Member; Sethu Raman, Committee Member; Gary Lackmann, Committee Member
Numerical Simulations of Gulf Stream Meanders and Eddies and Their Effects on the Cross-Shelf Transport off the Carolina Coast
(2006-10-24) Liu, Xiaoming; Lian Xie, Committee Chair
Gulf Stream meanders and eddies off the Carolina coast are studied using an ocean numerical model. Numerical simulation results show that the isobathic curvature in the vicinity of the Charleston Bump has great effect on the development and evolution of Gulf Stream meanders and eddies, and the formation of the Charleston Trough, a Gulf Stream meander that appears as a low pressure or depressed water surface region downstream of the bump, is the result of the combined effect of the Charleston Bump and the isobathic curvature in the region. The isobathic curvature plays a major role in enhancing the baroclinic and barotropic energy transfer rates, whereas the bump provided a localized mechanism to maximize the energy transfer rate downstream of the Charleston Bump. The effects of the Gulf Stream meanders and eddies on the cross-shelf transport near the Carolina coast are also studied in the ocean numerical model. There are two factors that affect the ocean circulations on the continental shelf off the North and South Carolina coast: wind and the Gulf Stream meanders and eddies. It is concluded that the combination of the wind effect and the effect of the Gulf Stream meanders and eddies caused the red tide bloom event in the North Carolina near shore waters in October 1987. Neither wind alone nor the Gulf Stream meander/eddy alone is sufficient to transport the passive tracer to the near-shore area.
Simulation of Ocean Circulation around the Galápagos Archipelago Using a Hybrid Coordinate Ocean Model (HYCOM).
(2010-05-17) Liu, Yanyun; John Morrison, Committee Chair; Lian Xie, Committee Chair; Fredrick Semazzi, Committee Member; Daniel Kamykowski, Committee Member